A targeted attack focused on user names, passwords and security questions of Apple accounts gave hackers access to nude photos of celebrities that were then leaked over the weekend on the Internet, the company said Tuesday.

None of the cases of leaked photos resulted from a breach in any of its cloud systems including iCloud or Find my iPhone, the company said in a statement.

“We are continuing to work with law enforcement to help identify the criminals involved,” Apple said.

The company advised users of its cloud software to use a strong password and enable two-step verification, a process that requires users to type in a numerical code sent to their mobile device after they’ve entered their user name and password.

Apple said it has been investigating for more than 40 hours the incident, which resulted in the release of private photos of a number of actresses and models, including movie star Jennifer Lawrence.

Naked images of the celebrities have already been posted across sites like Reddit and 4Chan.